1. Field of the Invention
The invention relates to the exhaust casing of a turbojet engine provided with hanging lugs for the attachment of the rear suspension structure by which the engine is suspended from a mast situated under the wing of an aircraft.
2. Summary of the Prior Art
The hanging of a turbojet engine under the wing of an aircraft is effected by means of a mast designed to ensure the mechanical transmission of the stresses between the engine and the aircraft structure. The turbojet engine is suspended from the mast at two points defining a front suspension and a rear suspension. The transmission of the thrust itself is effected by inclined thrust take-up bars situated on both sides of the engine.
The rear suspension structure normally comprises at least three outer hanging lugs or forks which are formed integrally with the upper part of the turbojet engine exhaust casing. These lugs permit the exhaust casing to be fixed to the lower ends of three suspension links which have their upper ends mounted on an arcuate hanging structure fixed directly under the mast serving to support the turbojet engine.
In the case where the exhaust casing has a cross-section in the form of a regular polygon the sides of which are defined by flat plates of the casing, the geometrical axes of the hanging lugs are normally situated at vertices of the polygon. This characteristic permits only tensile or compressive suspension stresses to be generated in the structure of the flat plates adjacent the hanging lugs. This enables the thickness of these plates to be given a minimum value which would be inadequate if the method of suspension were to bring about bending stresses.
The hanging lugs are attached to the lower ends of the suspension links by fixing axles oriented parallel to the geometrical axis of the turbojet engine. This leads to the formation of a scallop-shaped indented part in the outer surface of the turbojet engine exhaust casing in line with each of the hanging lugs. The depth of this indented part is all the greater as the loads to be transmitted are the more substantial, because the diameter of the securing axles must be increased.
The exhaust casing supports a central casing or hub by means of arms which are inclined relative to a radial direction and are connected to the exhaust casing at the vertices of the polygonal cross-section thereof. These arms define between them sectors in which the gases issuing from the turbojet combusion chamber are channelled. The indented parts formed in line with the hanging lugs thus have the effect of reducing the cross-sectional area of flow of the exhaust gases in the corresponding sectors. The disturbance thus generated relative to the theoretical flow stream of the gases in each sector results in a lowering of the turbojet output and an increase in fuel consumption. These effects are all the more marked as the load transmitted through each of the hanging lugs is increased.
Furthermore, the inclined arms supporting the central hub are of unequal lengths, depending on whether or not they are connected to the exhaust casing at the location of a hanging lug. This introduces an imbalance when thermal expansions occur at the various speeds of the turbojet engine.
To overcome these drawbacks the hanging lugs may be moved outward relative to the vertices of the polygonal cross-section of the exhaust casing, as illustrated in particular in document GB-A-1 506 952. However, the stresses created by the suspension of the turbojet then introduce localized bendings in the flat plates forming the exhaust casing, which requires the thickness of the plates to be increased and, consequently, increases the mass of the turbojet. Thus, a result is obtained which is the reverse of that being sought.